1. Field of the Invention
Machine dishwashing generally consists of a prerinse cycle, a main wash cycle, one or more intermediate rinse cycles, a final rinse cycle and a drying cycle. This applies both to domestic and to institutional dishwashing machines.
Hitherto, it has mainly been standard practice in the case of domestic dishwashing machines, hereinafter referred to as DDWM, to place the detergent in a dispensing box which is generally located in the door of the machine and which automatically opens at the beginning of the main wash cycle. The preceding prerinse cycle is completed without any active substance, i.e. solely with the cold inflowing tapwater.
In institutional dishwashing machines, hereinafter referred to as IDWM, the so-called precleaning zone corresponds in principle to the prerinse cycle of a DDWM. In dishwashing machines for large kitchens, the detergent added to the main wash zone carries over into the precleaning zone where it is used to support the removal of adhering food remains. Although there are IDWM where the precleaning zone is only fed with fresh water, a precleaning zone where detergent is added is more effective than precleaning with freshwater alone.
The principle by which the precleaning zone operates in IDWM has already been applied to DDWM, enabling detergents to be added during the prerinse cycle by introduction in tablet form and positioning of one or more suitable tablets, for example, in an unoccupied part of the cutlery basket or even elsewhere in the machine, so that they could act both during the prerinse cycle and in the actual wash cycle, i.e. could perform a dual function.
2. Discussion of Related Art
The use of such detergent tablets is described, for example, in DE 35 41 145 A1. The tablets in question are detergent tablets of uniform composition with a broad dissolving profile for machine dishwashing which contain typical alkaline-reacting components, more particularly from the group of alkali metal metasilicates and pentaalkali metal triphosphates, active chlorine compounds and tabletting aids, and in which the alkali metal metasilicates consist of a mixture of "sodium metasilicate nonahydrate" (Na.sub.2 H.sub.2 SiO.sub.4.8H.sub.2 O) and anhydrous sodium metasilicate while the pentaalkali metal triphosphate consists of anhydrous pentasodium triphosphate, the ratio by weight of anhydrous sodium metasilicate to sodium metasilicate nonahydrate being 1:0.3 to 1:1.5 and the ratio by weight of pentasodium triphosphate to sodium metasilicate--both anhydrous--being from 2:1 to 1:2 and preferably from 1:1 to 1:1.7.
Tablets such as these have such a broad dissolving profile that, even in the prerinse cycle of a DDWM, at least 10% by weight of the tablets can be dissolved by the cold inflowing tapwater, a pH value of at least 10.0 being developed in the wash liquor. Given high solubility in warm water, at least 60% by weight and preferably at least 70% by weight of the tablets are still available for the main wash cycle.
In the context of the invention, the dissolving profile is understood to be the ratio by weight of parts of the tablet dissolved under the conditions of the prerinse cycle of typical DDWM to the tablet as a whole.
However, known tablets contain phosphates which are known to be undesirable.
However, there are also commercially available phosphate-free detergent tablets for dishwashing machines (for example Hui Spul-Tabs, a product of Roth GmbH, Bad Ems) which essentially contain silicates, nonionic surfactants, organic complexing agents and percarbonate. However, when these tablets are placed in the machine (for example in the cutlery basket), they dissolve completely or substantially completely during the actual prerinse cycle, so that hardly any more detergent is available for the main wash cycle. In addition, the stability of these tablets is unsatisfactory.
DE 40 10 524 A1 describes stable, dual-function phosphate-free detergent tablets for dishwashing machines containing silicate, low-foaming nonionic surfactants, organic complexing agents, bleaching agents and water and, in addition, organic complexing agents according to DE 39 37 469 A1 in the form of a granular alkaline detergent additive consisting of sodium salts of at least one homopolymeric or copolymeric (meth)acrylic acid, sodium carbonate, sodium sulfate and water. In the production of these tablets, the granular alkaline additives are mechanically mixed with the other generally powder-form constituents and the resulting mixture is tabletted in known manner.
Now, the problem addressed by the present invention was to follow a market trend by providing a stable, dual-function, phosphate- and metasilicate-free low-alkaline detergent tablet with a broad dissolving profile for dishwashing machines, at least 10% by weight to about 40% by weight of which is dissolved by the cold tapwater flowing into the prerinse cycle of a DDWM, which develops a pH value of at most about 10.5 in the wash liquor and of which at least 60% by weight to around 90% by weight is still available for the main wash cycle by virtue of the high solubility in the tablet in warm water.
The known detergent tablets were produced by the tabletting of powder mixtures containing anhydrous sodium metasilicate in addition to sodium metasilicate nonahydrate containing water of hydration. This combination of water-containing substances and substances capable of absorbing water led to an increase in the resistance of the tablets to breakage in storage. Since the tablets according to the invention cannot contain any of the raw materials mentioned above in view of the low alkalinity required, they are not sufficiently resistant to breakage after tabletting in accordance with the prior art from powder mixtures or from powder mixtures with a granular component.